The present invention pertains to microlithography apparatus and methods as used in the manufacture of, e.g., semiconductor integrated circuits and display devices. More specifically, the invention pertains to such microlithography apparatus and methods that employ a charged particle beam (e.g., electron beam or ion beam) as an energy beam. Even more specifically, the invention pertains to charged-particle-beam (CPB) microlithography apparatus and methods exhibiting improved correction of astigmatisms that otherwise would cause linear distortion and degradation of edge resolution of the pattern image.
Conventional charged-particle-beam (CPB) microlithography methods and apparatus are exemplified by electron-beam microlithography methods and apparatus. Such conventional apparatus and methods embody any of several approaches. One approach, termed xe2x80x9ccharacter projectionxe2x80x9d exposure, is used for transferring patterns comprising pattern portions that are highly repeated (e.g., as found in DRAM patterns). In character projection exposure, the pattern image (defined by a reticle) is demagnified (xe2x80x9creducedxe2x80x9d) as the image is being projected onto the substrate (e.g., semiconductor wafer). The reticle defines the repeating portions of the pattern (for a DRAM, each repeating portion measures, e.g., about 5 xcexcm square on the substrate). The non-repeating portions of the pattern are separately written directly onto the substrate using another method such as the xe2x80x9cvariable-shaped-beamxe2x80x9d writing technique.
The character projection exposure technique provides a substantially improved xe2x80x9cthroughputxe2x80x9d (number of wafer substrates that can be processed per unit time) over the variable-shaped-beam writing technique. However, the throughput realized using the character projection exposure technique is too low, by at least an order of magnitude, to be practical for industrial-scale production of actual semiconductor integrated-circuit devices, including DRAMs.
A technique that has been proposed for improving throughput over the character projection exposure technique involves illumination of the entire reticle pattern for a semiconductor device onto the substrate in one exposure or xe2x80x9cshotxe2x80x9d of the charged particle beam. To project the (usually demagnified) image, a two-stage projection lens is used. An example of such a technique is discussed in Japanese Laid-Open (xe2x80x9cKokaixe2x80x9d) Patent Application No. Hei 5-160012.
Frequently, projection of the entire reticle pattern in a single shot cannot be realized due to practical constraints. For example, larger patterns require CPB optics having correspondingly larger fields; thus, the physical size of the CPB optics must be correspondingly larger. Simply increasing the size of the CPB optics as required for a larger pattern tends to introduce a myriad of other problems that generally degrade image resolution and projection accuracy.
To address such problems, the reticle pattern is divided into multiple subfields each defining a respective portion of the overall reticle pattern. As a result, the CPB optics can be much smaller than would otherwise be necessary for single-shot exposure of the entire pattern. Each subfield is typically sized and shaped to correspond to the size and shape of the field of the CPB optical system. The subfields are illuminated by the charged particle beam, and thus projected onto the substrate, individually one at a time in an ordered manner. The subfields are controllably projected onto the substrate such that each projected subfield is in the proper position and alignment, relative to the other projected subfields, to form the complete pattern on the substrate. As each subfield is illuminated, certain operational parameters of the CPB optical system can be changed as required from subfield to subfield to achieve improved image resolution and alignment relative to the other projected subfields. This technique of xe2x80x9cpartitioningxe2x80x9d the reticle pattern into individually projected subfields is disclosed, for example, in U.S. Pat. No. 5,260,151, incorporated herein by reference.
All the conventional CPB microlithography apparatus and methods summarized above tend to exhibit an unacceptable degree of linear distortion. Linear distortion can cause, for example, a square subfield to be imaged onto the substrate as a rectangle, or a parallelogram-shaped subfield to appear warped when projected onto the substrate.
Conventional electron-beam microlithography apparatus as used for variable-shaped-beam transfer methods employ an aperture to control beam shape and size. Stigmator lenses are used to reduce linear distortion of an image of the aperture. In a stigmator lens used in such apparatus, the lens center can be shifted as required within a plane perpendicular to the optical axis (i.e., shifted in the X and Y directions). The stigmator lens is usually configured as a pair of quadrupole coils that are rotationally displaced, about the optical axis, from each other by 45 degrees. Shifting of the lens center is performed by changing the magnetic-field strengths of the quadrupoles relative to each other.
Stigmators are generally used to reduce either or both of the following problems: astigmatism that would otherwise degrade the edge resolution of the reticle image, and linear distortion of the reticle image. Conventionally, correcting either of these problems tends to aggravate the other problem. As a result, in order to establish conditions adequate for satisfactorily reducing both problems, the corrections must be repeated many times. Hence, a large number of stigmators are conventionally required, which substantially increases cost and complexity, and imposes accompanying greater burdens of controlling the entire CPB optical system in a coordinated manner.
In view of the foregoing, an object the present invention is to provide charged-particle-beam (CPB) microlithography apparatus and methods offering precise yet simple correction of any of several types of astigmatisms, including astigmatism that would otherwise degrade edge resolution of the reticle image, and astigmatism that would otherwise cause linear distortion of the reticle image. It is a further object that such corrections be performed without the need to make the corrections multiple times.
To such ends, and according to a first aspect of the invention, CPB microlithography apparatus are provided that comprise an illumination-optical system for illuminating, with a charged particle beam, a pattern defined on a reticle. Such an apparatus also includes a projection-optical system for projection-imaging, on a sensitive substrate, a charged particle beam that has passed through the reticle. The projection-optical system comprises at least one stigmator lens each comprising first and second respective stigmator coils. Each stigmator lens has a principal plane aligned with a specific position on the optical axis of the projection-optical system. The principal plane is adjustable by setting or adjusting the ratio of magnetic-field strengths and convergence/divergence of the respective stigmator coils.
By way of example, and according to an especially advantageous embodiment, the first stigmator coil is energized to produce a magnetic-field strength denoted by A1, and the second stigmator coil is energized to produce a magnetic-field strength denoted by A2. Also, the first stigmator coil is located an axial distance a, from the principal plane of the stigmator lens, and the second stigmator coil is located an axial distance a2 from the principal plane of the stigmator lens. The respective axial distances and magnetic-field strengths are related according to a1A1=a2A2.
The foregoing and additional features and advantages of the invention will be more readily apparent from the following detailed description, which proceeds with reference to the accompanying drawings.